This invention relates to a valve disc and drive shaft assembly particularly for a butterfly valve.
In a known form of butterfly valve, the disc is cast to its general shape and then a diametrically arranged bore is formed by drilling. This bore extends across the whole diameter of the disc and receives the drive shaft by means of which the disc is rotated to open or close the valve. To prevent the shaft rotating in the bore one or more pins are provided radially with respect to the shaft axis and inserted through holes in the disc into blind apertures on the shaft.
However, it has been found that sometimes the joint between a pin and a disc is not properly formed or becomes damaged and leakage occurs either through the disc from one side to the other, or from the interior to the exterior of the valve along a leakage path alongside the drive shaft.
When in the closed condition the disc of a butterfly valve disc is often subjected to a considerable pressure caused by the fluid acting on one side. As a result the disc may bend and very high bending movements are applied to the shaft. To open the valve a large torque has to be applied to the shaft and additionally in order to support these loads the shaft has to be of relatively large diameter.